In a full dimensional multi-input multi-output (FD-MIMO) communication system 100, e.g. as illustrated in FIG. 1, a 2-dimensional planar antenna array structure 101 may be used in which the antenna elements 104 may be placed in the vertical and horizontal direction. Each antenna element 104 generates a beam 102 for transmitting a radio signal to a mobile device or user equipment (UE) 103. The radio signal may be transmitted to the UE 103 by using more than one antenna elements 104 of the antenna array 101.
The total antenna number may exceed the number of eight, which is the maximum antenna port number supported in some mobile standards such as LTE up to Release 12 by the Channel State Information Reference Signal (CSI-RS). As a result of this CSI-RS antenna port number limitation, designing the closed loop CSI feedback for the FD-MIMO system becomes problematic.